1. Field of the Invention
The present invention relates to an alcohol-based flexographic ink for use in colorless backcarbon papers.
2. Description of the Prior Art
In general, backcarbon papers are papers prepared by locally coating only the required parts of the back side of a thin form paper with a hot melt type ink having a dark color of indigo, black, red or the like by the use of a letterpress printer or a gravure printer. When the back side (coated side) of a backcarbon paper is superimposed on plain paper and manual printing or typewriter printing is applied on the front side of the backcarbon paper, a transferred inage is formed on the plain paper.
In these backcarbon papers, because the fairly large part of their back side is coated with an ink of dark color, the ink can be seen through even at the front side. This makes it difficult to read letters printed on the front side and gives an unpleasant feeling.
On the other hand, so-called carbonless papers are colorless before use and, by applying manual printing on these papers, there is formed a transferred image of blue, black, red or the like on a plain paper. Hence, in Japan, as form papers, carbonless papers are now in wider use than backcarbon papers.
However, in carbonless papers, it is generally necessary to prepare three kinds of papers, namely, an upper paper coated at the whole part of the back side with microcapsules containing an oil droplet containing a colorless dye (electron-donating organic color former), a lower paper coated at the whole part of the front side with an electron-accepting organic color developer and a middle paper having two functions of the upper and lower papers. Moreover, it is necessary at the time of actual form paper production to apply to parts requiring no copying a treatment for prevention of copying called "desensitization printing", which is not desirable from the standpoints of productivity of form paper production and energy saving.
Hence, there has been desired a copying paper having only respective advantages of a backcarbon paper and a carbonless paper, namely, a paper not requiring a combination of an upper paper, a middle paper and a lower paper and locally coated with a colorless ink only at the required parts of the back side.
Such a colorless backcarbon paper was proposed by the present inventor in Japanese Laid-open Patent Publication No. 203588/1982 (title of the invention: Carbonless Pressure-Sensitive Transfer Sheet). This backcarbon paper of prior art is obtained by coating the whole surface or part of a substrate sheet with a flexographic ink produced by dispersing and/or dissolving (A) microcapsules containing an oil droplet containing an electron-donating organic color former, (B) microcapsules containing an oil droplet containing an electron-accepting organic color developer and (C) a binder, in a solvent composed mainly of a low boiling alcohol of 1 to 3 carbon atoms. When a pressure is applied to the front side of the above backcarbon copying paper, the two kinds of the colorless microcapsules (A) and (B) on the backside of the copying paper are destroyed and the two kinds of the oil droplets contained in the microcapsules (A) and (B) come in contact with each other developing a color, and the color is transferred onto a plain paper placed beneath the copying paper whereby a transferred image is formed on the plain paper. However, further improvement of the density of this transferred image has been desired.